In recent years, flash memory devices have been widely used in computer related equipment and other electronic appliances as storage devices. The nonvolatile and in-system programmable capabilities of a flash memory are very important for storing data in many applications. As an example, flash memories are frequently used for the BIOS storage of a personal computer. In addition, the small physical size of flash memories also makes them very suitable for portable applications. Therefore, they have been used for storing programs and data for many portable electronic devices such as cellular phones, digital cameras and video game platforms. It becomes more and more important that the flash memory devices can be used in portable systems that have only low voltage power supply.
An EPROM type flash memory uses Fowler-Nordheim (F-N) tunneling effect to erase memory cells' threshold voltages (Vts) to a low state, and uses channel-hot-electron (CHE) injection to program cell' Vts to a high state. Traditionally, the cell array is a NOR-plane array. Therefore, a higher threshold voltage Vt(0) is programmed for data "0" and a low threshold voltage Vt(1) is for data "1". The threshold voltage Vt(1) in the low state is in the range of about 0.5 volts to 3 volts. Vt(1) below 0.5 volts is not permitted to avoid the sub-threshold leakage induced by the coupling of the bit line voltage to the memory cell's floating gate during read and program operations. When many memory cells are connected to the same bit line, the overall threshold leakage of the selected bit line can be greater than 100 uAs that may result in false reading or greater than several mAs that may overload the on-chip pump in the program operation. In the high state, the threshold voltage Vt(0) is usually greater than 6.5 volts.
Because of the wide threshold voltage distribution between low and high states, the gate (word line) voltage has to be about 5 volts in a memory read operation in order to distinguish the two states regardless of the power supply condition. If the power supply voltage Vdd is greater than 5 volts, the gate voltage can be easily applied. However, many portable systems now have only low voltage power supply with Vdd ranging from 2.0 volts to 5.0 volts to reduce power consumption. Under the low Vdd condition such as 2 or 3 volts, it is necessary to boost the voltage to around 5 volts so that the word line of the flash memory can be biased properly in a read operation. The voltage boosting circuit is fairly complicated and requires much device area. It becomes a significant drawback for a portable system to use such flash memories.
Data retention is another major problem that conventional flash memories have to be concerned with. Traditionally, a flash memory cell is manufactured with a threshold voltage around 2 volts after UV (ultra-violet) erasure. UV erased threshold voltage Vt(UV) represents the threshold voltage when there is no electrons in the floating gate of the memory cell. It is known that the electrons being stored in the floating gate by programming a flash memory cell to a high state may eventually move away and reduce the threshold voltage to Vt(UV) after a long period of time. This would cause the data retention problem. Data retention is a concern as long as the high state voltage Vt(0) is much greater than Vt(UV). As it is well known, the higher the voltages of Vt(0) and Vt(1), the sooner the memory decaying speed. Typically, 1 volt of Vt drop from 6 volts to 5 volts is ten times faster than 1 volt drop from 5 volts to 4 volts. Therefore, data retention is less a concern for Vt(1) than Vt(0). It is desirable that both Vt(0) and Vt(1) be kept as low as possible as Vdd is scaled down to reduce the Vt decaying speed and thus solves the data retention problem.
In the traditional NOR-plane flash memory array, however, lowering Vt(0) and Vt(1) would result in higher possibility of over-erasure that ends up with negative Vt(0) and Vt(1). Therefore, Vt(0) and Vt(1) can not be much reduced as Vdd is lowered. In other words, in the conventional flash memory, Vt(0) is still defined to be much higher than Vt(UV) and Vt(1) is near or below Vt(UV) but can not be the ground level when Vdd is between 2 volts and 5 volts.